Because they were going to be a puppet. If Cam stays he needs to manage the team, and let his coordinators run the show, but he needs to hold them accountable, and step in when need be.
To be fair, one should add that Cam basically wanted a lame duck as OC as he was about to call his own plays anyway. That kind of limited the field of candidates dramatically. If there's one aspect of the ever popular Cam bashing I can somewhat relate to it's that he tries to do too much on offense (resulting in bad clock management and some high risk calls) and not enough on other aspects of the game (neglecting the special teams, for instance). In the long run, it might be wise to give the playcalling duties to someone else and focus on being the headcoach.
